The present invention relates to a titanium dioxide pigment which, in order to improve its dispersibility, is coated with an inorganic substance, and to a process for coating titanium dioxide pigment with an inorganic substance.
Titanium dioxide pigments are used in many fields, for example in the paint, plastics and rubber industries. The effectiveness of the pigment used depends to a large extent on how evenly the pigment particles can be dispersed into the product. It is generally known that the wetting dispersing properties of titanium dioxide pigments can be improved by depositing inorganic metal oxide and/or metal hydroxide coatings on the surface of titanium dioxide crystals. It is also known that, by treating titanium dioxide pigments with organic compounds, the dispersing properties of the pigments can be further improved. The use of the pigment determines the after-treatment which is most advantageous.
In extrusion coating, paper, cardboard, laminate or the like is coated with a thin pigmented plastic film. In extrusion coating, the temperature of the molten plastic is 300.degree.-320.degree. C. If a titanium dioxide pigment coated with an inorganic substance is used for pigmenting plastic, the water chemically bound in the inorganic coating is released at a high temperature, whereby holes are formed in the coating film (so-called lacing phenomenon). In extrusion coating, the thickness of the plastic film is 25-50 .mu.m and its TiO.sub.2 concentration is 10-15%.
High requirements are set on the dispersibility of pigment, since in thin plastic films the undispersed pigment agglomerates show as spots, and unevenly dispersed pigments result in surface of irregular color. Furthermore, in most cases the optimum effect of the pigment is not utilized to the full extent. Since a very good dispersibility is required of the pigment, inorganically after-treated pigments should be used in extrusion coating, but owing to the lacing phenomenon, nowadays only titanium dioxide pigments which have not been inorganically after-treated are used in extrusion coating.
The object of the present invention is to provide a titanium dioxide pigment coated with an inorganic substance, eliminating the above-mentioned disadvantages.